Light-emitting diodes (LED) are semiconductor light sources which consume less power and last longer than traditional incandescent light bulbs. LEDs can be manufactured to produce ultraviolet, infrared, or visible light. LEDs produce light when a current is allowed to flow from the positive anode to the negative cathode across a semiconductor, displacing electrons which subsequently release photons. LEDs can also be made much smaller than traditional light sources, allowing them to be placed on substrates such as printed circuit boards (PCBs). Elements in electronic circuits can be either reversible or irreversible with respect to current flow. Resistors are an example of a reversible, or orientation-agnostic element, which operates essentially identically regardless of current direction. Diodes including LEDs are irreversible, or orientation-specific elements, which operate differently, or not at all, if the current direction changes.